1. Field of the Invention
The present invention relates to improvements in a brake booster for use in, for example, an automobile, and, more particularly, to a vacuum brake booster or pneumatic brake booster comprising a power cylinder combined with a hydraulic cylinder and a wear detector to be operated by a stroke of a power piston associated with the power cylinder for detecting the wear of a brake lining.
2. Description of the Prior Art
FIGS. 1 and 2 show an example of the conventional brake system for use in a four-wheel vehicle which includes a pair of brake boosters 1 and 1', each of an air over hydraulic type. Since the brake boosters 1 and 1' are the same in construction and operation, the booster 1 only for front wheels 8 will be described. The other brake booster 1' is applied to the rear wheels 9 and connected with the front wheel brake booster 1 in parallel.
In FIG. 1, the brake booster 1 is constituted of a power cylinder 2 combined with a hydraulic cylinder 3. The power cylinder 2 is communicated with an air reservoir 5 through a relay valve 4.
The relay valve 4 is controlled in response to an operation of a braking valve 6 so that when the latter is opened and closed the former 4 will be opened and closed correspondingly. Compressed air is supplied to the air reservoir 5 by means of an air compressor 7 and reserved in the reservoir. When an operator of the vehicle provided with the brake boosters 1 and 1' steps on a brake pedal 6a, a pressure indication is transmitted from the reservoir 5 to the relay valve 4, causing the latter to be opened. With the relay valve 4 being opened, compressed air is supplied from the air reservoir 5 to the power cylinder 2 for its operation. Consequently, the hydraulic cylinder 3 is operated to deliver a braking fluid, under pressure, to the front wheels 8.
FIG. 2 is a cross-section of the brake booster 1 and will be described in more detail with reference to the drawing. A pressure indication caused by the stepping-in of the pedal 6a, is supplied to a pressure indication port 11 of the relay valve 4. This causes a relay piston 12 to be lowered and to press a valve disc 13 down. With the valve disc 13 being pressed down, a supply port 14 which is in communication with the air reservoir 5, comes in communication with an outlet port 15 which is in communication with the power cylinder 2 so that compressed air from the air reservoir 5 is permitted to pass through the supply port 14, the outlet port 15 and a passage 16 to a pressure chamber 17 of the power cylinder 2. Thus, the power piston 18 of the power cylinder 2 is moved in the direction shown by the arrow T.sub.1. A rod 19 of the piston 18 is connected to a hydraulic piston 20 of the hydraulic cylinder 3. With the movement of the power piston 18, the hydraulic piston 20 is also moved in the direction shown by the arrow T.sub.2, so that brake fluid is supplied from a fluid pressure discharge port 21 of the hydraulic cylinder to the front wheels 8. An atomospheric chamber 22 of the power cylinder 2 is in communication with the atmosphere through a port 23. The relay valve 4 is formed with an exhaust port 24 to exhaust compressed air from the pressure chamber 17 when the braking force is to be removed. Reference numeral 25 shows an oil chamber which is connected to an oil reservoir 26 in FIG. 1.
The booster 1 is further provided with a wear detector 27 for detecting the wear of the brake lining. The purposed of the wear detector 27 is to notify the operator that the amount of wear of the brake lining has reached a predetermined level.
The wear detector 27 is constituted of a switch operating member 28 movable with the movement of the power piston 18 and an electric switch 29 to be operated when it is lowered by the movement of the operating member 28.
The principle of operation of the wear detector 27 is as follows: When the amount of wear of the brake lining is zero, the power piston 18 is stroked or moves for a distance l and, at the end of the stroke shown by a line A, a braking effect occurs. With increase of the wear amount of the brake lining, the stroke of the power piston 18 needed to actuate a braking effect will be increased. For example, the brake may be actuated only when the power piston 18 is stroked for a distance l.sub.1 and reaches a line B. The difference l.sub.d between the strokes l and l.sub.1 corresponds to the wear amount of the brake lining. Therefore, by positioning the top of the switch operating member 28 at the distance l.sub.d corresponding to a predetermined wear amount, the power piston 18 may push the switch operating member 28 in the direction T.sub.1 when the wear amount of the brake lining exceeds a predetermined level to close an electric switch 29 engageable by a recess 30 formed in the member 28, so that the operator becomes aware that the wear amount of the brake lining exceeds the predetermined level.
On the other hand, even when the wear amount of the brake lining is the same, in the case of mild braking, the stroke of the power piston 18 may be shorter than that in the case of abrupt braking. As shown in FIG. 3, when the wear amount of the brake lining is zero, the power piston 18 may be moved by a distance l with a mild braking and for a distance l.sub.2 with abrupt braking. With a predetermined wear amount of the lining being reached, the power piston 18 may be stroked for a distance l.sub.1 with mild braking and for a distance l.sub.3 with abrupt braking. FIG. 4 is a graph showing the above conditions. In FIG. 4, the ordinate shows the stroke of the power piston 18 and the abscissa shows the operating pressure of the power piston, i.e., braking force. The difference in stroke between mild and abrupt brakings means the ultimate difference in power piston operating pressure therebetween and, thus, the more abruptly the braking operation is applied, the larger will be the power piston operating pressure. In proportion to the power piston operating pressure, the braking fluid pressure of the wheel cylinder will be increased. Accordingly, as the transformation of the brake shoe and brake drum becomes large, the stroke of the power piston will be increased to that extent. Namely the relation between the power piston operating pressure and the power piston stroke with the wear amount of the lining being zero is shown by a line C and that with the wear amount of the lining being a predetermined level is indicated by a line D.
In order to provide an accurate wear detecter for the brake lining, it should be constructed so that the switch 29 can be actuated in a hatched area E above the line D. On the other hand, since the conventional wear detector is constructed so that the switch 29 is actuated at a constant stroke of the power piston, the conventional wear detector has no mechanism for making the stroke amount variable when the switch 29 is actuated. Therefore, for example, when the constant amount of the stroke at which the switch 29 is actuated is set at F, the switch 29 may not be actuated in a hatched zone G defined by an area below the line F and above the line D where the wear reaches the predetermined level while the switch 29 may be actuated in a zone H defined by an area above the line F and below the line D where the wear is still below the predetermined level.
In the conventional lining wear detector, the stroke of the power piston, when the switch 29 is actuated, is set on a line I, i.e., at a level of value determined by the predetermined wear and the maximum power piston operating pressure (maximum abrupt braking). With this setting of the stroke of the power piston, it is possible to overcome the disadvantage that the switch 29 may be actuated before the wear reaches the predetermined level as in zone H. However, there is still a disadvantage that, in a zone J defined by an area below the line I and above the line D, the switch 29 may not be actuated regardless of whether or not the wear has reached the predetermined level. Moreover, when the braking operation is allowed to continue within the range of low operating pressure of the power piston, there occurs the possibility that the lining will be worn out to exceed a predetermined extent by a maximum wear amount corresponding to the stroke L.
When the maximum abrupt brake (using the maximum power piston operating pressure) is applied with the lining being thus excessively worn, the piston stroke may increase along a line M up to a point N. That is, if the detector is so constructed that, when the wear of the lining reaches the predetermined level, the switch 29 is actuated regardless of the power piston operating pressure, the range of the stroke of the power piston is up to a point O. However, in the conventional detector, there is a possibility that the stroke range is up to the point N. Therefore, it is necessary to provide an allowance stroke L corresponding to the distance between the points O and N as shown in FIG. 3, which will necessitate the detector to be bulky in structure.
In summary, the conventional lining wear detector is constructed such that the switch is actuated when the power piston reaches a single predetermined position. Therefore, it is impossible to exactly detect that the wear of the lining has reached a predetermined level. Further, owing to this disadvantage it is necessary to provide an allowance stroke for the power piston, causing the booster to be bulky.